With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer by bringing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive particles, such as silica (SiO2), onto the polishing surface.
A polishing apparatus for performing CMP has a polishing table that supports the polishing pad thereon, and a substrate holding apparatus, which is called a top ring or a polishing head, for holding a substrate such as a wafer. In the case where the substrate is polished using such polishing apparatus, the substrate holding apparatus holds the substrate and presses it against the polishing surface of the polishing pad at a predetermined pressure. At this time, the polishing table and the substrate holding apparatus are moved relative to each other to bring the substrate into sliding contact with the polishing surface to thereby polish a surface of the substrate.
When polishing the substrate, if a relative pressing force applied between the substrate and the polishing surface of the polishing pad is not uniform over the entire surface of the substrate, insufficient polishing or excessive polishing would occur depending on the pressing force applied to each portion of the substrate. Thus, in order to uniformize the pressing force applied to the substrate, the substrate holding apparatus has a pressure chamber formed by a membrane (elastic membrane) at a lower part thereof. This pressure chamber is supplied with a fluid, such as air, to press the substrate through the membrane under a fluid pressure.
However, because the above-described polishing pad has elasticity, the pressing force becomes non-uniform in an edge portion (peripheral portion) of the substrate during polishing of the substrate. Such non-uniform pressing force would result in so-called “rounded edge” which is excessive polishing that occurs only in the edge portion of the substrate. In order to prevent such rounded edge, a retaining ring for retaining the edge portion of the substrate is provided so as to be vertically movable relative to a top ring body (or carrier head body) and to press the polishing surface of the polishing pad located at the outer circumferential edge side of the substrate.
In the above-described polishing apparatus, because a frictional force is generated between the substrate and the polishing pad during polishing, this frictional force is received by the retaining ring to prevent the substrate from being slipped out of the lower part of the top ring body. Further, as described above, the retaining ring presses the polishing pad to deform the polishing pad, so that the polishing amount of the edge portion (peripheral portion) of the substrate is controlled by the deformation of the polishing pad.
When the substrate is polished under high frictional forces developed between the substrate and the polishing pad and at a low relative speed between the substrate and the polishing pad, the substrate holding apparatus tends to vibrate due to stick-slip, etc. The substrate which is being polished may be slipped out of the top ring by vibrations caused under such strict polishing conditions. Since the polishing of the substrate in such a vibrational range should be avoided, the actual range of polishing conditions is narrower than those that can be established by the polishing recipe. Attempts to reduce the unpolishable range increase the degree of freedom for combination of recipes, leading to improved polishing performance.
According to a study conducted by the inventors of the present invention, the source of the vibrations is considered to lie between the polishing pad and the substrate, and it is considered that the vibrations of the substrate are transmitted through the retaining ring to the top ring body and the pressing by the retaining ring becomes unstable due to complex factors such as resonance of the top ring, and thus a gap is formed between the retaining ring and the polishing pad to cause the substrate to be slipped out of the top ring during polishing.